1. Field of the Invention
The present invention relates generally to a mobile communication system supporting a multimedia service including voice and data services, and in particular, to a power controlling method for a terminal located in a soft handoff region.
2. Description of the Related Art
A typical mobile communication system, for example, a CDMA (Code Division Multiple Access) mobile communication system such as IS-2000 supports only voice service. Recently, however, communication technology has been developed to support data. HDR (High Data Rate) is a mobile communication system proposed to support only a high rate data service.
Although there is a need for supporting concurrent provision of voice and data services, existing mobile communication system support them separately. To satisfy this demand, a standardization called 1×EV-DV (Evolution-Data and Voice) system has been proposed recently.
In mobile communication, a whole service area is segmented into cells, and the cells are managed by base stations (BSs). By controlling the BSs under an MSC (Mobile Switching Center), mobile stations (MSs) are allowed to continue calls, moving from one cell to another cell. A BS communicates with an MS on a radio channel. As such, when compared to land communication systems that communicate by cable, mobile communication systems may experience transmission errors due to fading and interference. The most popular method of preventing the transmission errors is to use power sufficiently high to cover fading and interference. However, excess power may interfere with radio channels of adjacent users. Therefore, power control of a radio channel significantly influences system performance. Generally, a BS and an MS perform mutual power control. A procedure of controlling the power of a forward channel is called a forward power control and a procedure of controlling the power of a reverse channel is called a reverse power control.
A CDMA mobile communication system connects a plurality of code channels simultaneously on a single frequency channel at the same time point. Relying on this characteristic, an MS in an overlap region between BSs can communicate with the BSs concurrently on radio channels for a call. This is a soft handoff. At a soft handoff, power control must be performed for all the BSs communicating with the MS.
FIG. 1 illustrates channels associated with forward and reverse power control when an MS implements a soft handoff in a conventional mobile communication system. Here, the MS communicates with two BSs during the soft handoff.
Referring to FIG. 1, for a reverse power control, each of the BSs (sectors in the case of a sectored BS) compares the signal to noise ratio (SNR, that is, a pilot channel power to noise power ratio, Ep/Nt) of an R-PICH (Reverse Pilot Channel) from the MS with an outer loop set point preset for an outer loop power control. If the SNR is higher than the outer loop set point, the sector commands the MS to decrease reverse transmission power via an F-CPCCH (Forward Common Power Control Channel). If the SNR is less than or equal to the outer loop set point, the sector commands the MS to increase the reverse transmission power via the F-CPCCH.
At the soft handoff, F-CPCCHs (CPCCH1 and CPCCH2) are connected between the MS and at least two sectors (sector 1 and sector 2). If at least one of the F-CPCCHs commands a power decrease, the MS decreases its transmission power. The transmission power is increased only when the F-CPCCHs all command a power increase.
For a forward power control, the transmission power of the F-CPCCHs is determined by channel quality information received on an R-CQICH (Reverse Channel Quality Indicator Channel). The MS reports the reception strength of an F-PICH (Forward Pilot Channel) from a particular sector, that is, the carrier to interference ratio (C/I) of the F-PICH to the sector via the R-CQICH.
At the soft handoff illustrated in FIG. 1, the MS measures the C/I of an F-PICH from each of sector 1 and sector 2 and transmits the higher C/I on the R-CQICH to the sector having the higher C/I, that is, sector 1 in FIG. 1. Sector 1 determines the transmission power of CPCCH 1 using its C/I.
There are two problems that arise from the above described forward and reverse power control at the soft handoff.                (1) The first problem relates to a forward power control of at least two CPCCHs. An MS connects the F-CPCCHs to at least two sectors at a soft handoff, but the MS reports the C/I of a PICH from only one of the sectors. For example, if the MS is in communication with sector 1 and sector 2 at the soft handoff and sector 1 has better forward channel quality than sector 2 in FIG. 1, the MS transmits only the C/I of PICH 1 from sector 1 to sector 1. Sector 1 then determines the transmission power of CPCCH 1 using the C/I. On the other hand, sector 2 cannot determine the transmission power of CPCCH 2 because it does not receive the C/I of PICH 2.        (2) The second problems relates to determination of the transmission power of the R-CQICH. The transmission power of the R-CQICH stays constant at a predetermined ratio to that of an R-PICH and a reverse traffic channel. This implies that as the transmission power of the R-PICH and the reverse traffic channels decreases/increases, the transmission power of the R-CQICH must decrease/increase at the same rate.        
However, the R-CQICH is not soft-handed off like the R-PICH. Specifically, the R-CQICH is transmitted to only one sector having the best forward channel quality. Conversely, the R-PICH and the reverse traffic channel are transmitted to at least two sectors at a soft handoff. Thus, their reception performance is ensured. The reception performance of the R-PICH can be improved by selection diversity or combining.
If the same power control as for the R-PICH and the reverse traffic channel is applied to the R-CQICH at the soft handoff, a desired reception performance can be achieved from the R-PICH and the reverse traffic channel, but the reception performance of the R-CQICH may be lower than intended.